In recent years, liquid crystal display devices have frequently been used as display devices of information appliances such as a notebook type personal computer and a word processor and display devices of imaging appliances such as a mobile television, a portable motion picture camera, and a car navigation system owing to its characteristics such as light weight, thin thickness, and low power consumption. In the liquid crystal display devices, generally, a display panel is illuminated with light which is emitted from the rear by an internal lighting unit so as to realize a luminous display screen.
An edge light type lighting unit wherein a light guiding plate is disposed on the rear face of the display panel and a linear light source such as a fluorescent discharge tube is provided at a facet of the light guiding plate is ordinarily used as a backlight of the liquid crystal display device included in the notebook type personal computer and so forth owing to its characteristics of thin thickness and excellent uniformity in luminance on a light emission face.
A conventional edge light type lighting unit UT having fluorescent discharge tubes is shown in FIG. 6, and a liquid crystal display device LD having the lighting unit UT of FIG. 6 is shown in FIG. 7. The lighting unit UT has a tabular transparent light guiding plate 1 for transmitting light, fluorescent discharge tubes 2 separately disposed at two opposing sides out of four sides of the light guiding plate 1, and a reflection sheet 3 for reflecting light emitted from the fluorescent discharge tubes 2 to guide the reflected light to facets D1 of the light guiding plate 1. The facets D1 of the light guiding plate 1 are facets at which the fluorescent discharge tubes 2 are provided, and a facet D2 is a facet at which no fluorescent discharge tube is provided. The rear face and the facets D1 and D2 of the light guiding plate 1 as well as the fluorescent discharge tubes 2 are covered with the reflection sheet 3 to form a lighting element 10. This lighting element 10 is supported by frame portion 9 from its upper side and lateral sides as well as by a box-like rear face cover 8 fitted with the frame portion 9 from its lower side, so that the lighting element 10 is housed in a housing 30 formed of the frame portion 9 and the rear face cover 8. Light correction sheets 4 and 5 are provided on a light emission face of the light guiding plate 1. The lighting unit UT is constituted as described above. Then, a liquid crystal panel 11 and a front cover 12 are mounted on the light emission face of the lighting unit UT. The liquid crystal display device LD is thus constituted.
The reflection sheet 3 reflects light leaking from the light guiding plate 1 to the outside of the unit to return the leaked light to the light guiding plate 1, thereby increasing illumination light to be emitted from the light emission face. A white resin film having a high reflectance is used as the reflection sheet 3. Patterns (not shown) which makes a light diffusion area increased in proportion to distance from the fluorescent discharge tubes 2 serving as the light sources may be printed on the reflection sheet 3. As shown in FIG. 5, the reflection sheet 3 is cut to a predetermined shape so as to cover the rear face and the facets D1 and D2 of the light guiding plate 1 as well as the fluorescent discharge tubes 2, and perforations S are formed on predetermined positions so that the reflection sheet 3 is folded along the rear face and the facets.
Though a periphery of each of the fluorescent discharge tubes 2 (hereinafter referred to as “reflecting portion” 20) and the rear face of the light guiding plate 1 are covered with the reflection sheet 3 in this constitution, the reflection sheet 3 may be divided into sheets for separately covering the reflecting portion 20 and the rear face of the light guiding plate 1 with the divided sheets being adhered to one another with an adhesive double coated tape. The reflection sheet 3 shown in FIG. 6, which continuously covers the reflecting portion 20 and the rear face of the light guiding plate 1, is advantageous in achieving the thin thickness of the lighting unit UT and reducing the cost and the number of steps for assembly of the lighting unit UT.
A diffusion sheet, a prism sheet, and the like may be used as the light correction sheets 4 and 5. The light emitted from the light guiding plate 1 can be diffused or condensed by selecting a sheet to be used for the light correction sheets 4 and 5 among a various types of sheets varied in optical characteristics and by setting the number of sheets as required, thereby achieving uniformity and high luminance of the emitted light.
Each of ends of the fluorescent discharge tubes 2 is connected to lead wires (not shown) by soldering or the like, and the lead wires are connected to a power unit (not shown) such as an inverter for generating a high frequency alternating current. During operation of the lighting unit UT, a voltage required for lighting the fluorescent discharge tubes is applied from the power unit through the lead wires. Generally, a high voltage is required for lighting the fluorescent discharge tubes 2. Accordingly, a rubber holder 7 made from an insulator such as a rubber is attached to each of the fluorescent discharge tubes 2 and around the lead wires so as to protect electrodes of the fluorescent discharge tubes 2 and ensure safety of the lead wires by covering exposed portions thereof.
A plurality of heat discharge holes 15 for discharging heat generated from the fluorescent discharge tubes 2 are formed on the bottom of the box-shaped rear face cover 8. As described in the foregoing, the rear face cover 8 is fitted with the frame portion 9 so as to form the housing 30 for housing the lighting element 10.
The conventional lighting unit UT and the liquid crystal display device LD of the above constitution have gaps 16A formed between the rear face cover 8 and the frame portion 9 fitted with each other. Further, the lighting unit UT and the liquid crystal display device LD have gaps 16B formed between the rear face cover 8 and the lighting element 10. The gaps 16A and 16B are formed due to the constitution of itself. Dust entered the device during and after the assembly of the liquid crystal display device LD enters a display area 25 formed of the liquid crystal panel 11 and the light correction sheets 4 and 5 through the gaps 16A and 16B as well as gaps between the frame portion 9 and the lighting element 10. Also, dust entered the device through the heat discharge holes 15 formed on the rear face cover 8 enters the display area 25 through the gaps 16B and the gaps between the frame portion 9 and the lighting element 10. The dust entered the display area interrupts the output light to cause irregular luminance. Further, the components in the display area 25 such as the light correction sheets 4 and 5 are damaged due to friction with the dust. Moreover, when the dust enters a gap between the liquid crystal panel 11 and the lighting unit UT, it is very difficult to remove the dust without disassembling the display device. Thus, a precaution against the dust entrance is an important issue in maintenance of the liquid crystal display device LD after the assembly.
Further, when the heat generated from the fluorescent discharge tubes 2 during the operation of the lighting unit UT is discharged through the heat discharge holes 15 to outside, the air in the gaps 16B between the rear face cover 8 and the lighting element 10 functions as a heat insulation layer to cause insufficient heat discharge, thereby causing various defects attributable to the heat.